PHOENIX (AP) — A Phoenix racetrack’s horses remained under quarantine Friday as officials worked to get the upper hand on an equine herpes outbreak that has infected more than a dozen horses in New Mexico.

One horse had to be euthanized this week at Turf Paradise in north Phoenix, manager Vincent Francia said. The horse was one of three that arrived Jan. 20 from Sunland Park, New Mexico.

The other two are under isolation for 21 days. So far, they show no signs of the virus.

When the facility first learned horses in Sunland Park were testing positive for the virus, the three horses were tested. Francia said a nasal swab came back negative for all three and the animals seemed fine. Then one, a mare, was suddenly unable to stand Wednesday. She eventually had to be put down.

The horse’s body was sent to a lab in Tucson for testing. Though the results won’t be available for a week, Turf Paradise staff immediately enacted measures to disinfect anything that comes in contact with the horses.

“We got very aggressive,” Francia said. “This virus is so easily transferable from human to horse.”

Workers have been sanitizing everything in stable stalls including the walls, tack supplies and even water buckets. Even the starting gates have had to be cleaned. Jockeys will have to use Chlorox wipes on their hands and disinfect their riding boots after each race. Only essential personnel such as veterinarians will be allowed in the stables, Francia said.

Turf Paradise currently houses 1,700 horses and will continue to hold races as scheduled. However, no horses will be allowed to leave or enter the facility for the duration of the quarantine.

Even if nothing turns up under the quarantine, Turf Paradise will likely not accept horses from New Mexico for the foreseeable future.

“It’s not because we’re trying to be tough with New Mexico,” Francia said. “But the situation (in Sunland Park) on a daily basis keeps getting worse.”

Officials at Sunland Park Racetrack and Casino say they have temporarily suspended all races because of the outbreak. So far, 18 horses have tested positive for the equine virus. Of those animals, one was euthanized.